Turbo charging an engine allows the engine to provide power similar to that of a larger displacement engine while engine pumping work is maintained near the pumping work of a normally aspirated engine of similar displacement. Thus, turbo charging can extend the operating region of an engine. Turbocharged engines utilize a turbocharger to compress intake air and increase the power output of the engine. A turbocharger may use an exhaust-driven turbine to drive a compressor which compresses intake air. As the speed of the compressor increases, more boost is provided to the engine. However, during certain vehicle launch conditions, such as when accelerating from idle, due to minimal exhaust gas flow combined with increased load on the compressor, it may take an amount of time for the turbine and compressor to speed up and provide the required boost. This delay in turbocharger response, termed turbo lag, may result in a delay in providing the demanded engine power.
One example approach to reducing turbo lag is shown by Pallett et al. in U.S. Pat. No. 8,355,858. Therein, during deceleration and/or idle conditions when a driver tip-in is imminent, a first and second fuel injection amount is utilized, where the first amount produces a lean combustion and sufficient torque to maintain engine speed, while the second injection injected after the lean combustion provides additional exhaust reductant to match excess air of combustion, and generate sufficient exhaust energy to maintain the turbocharger speed at a target speed.
However, the inventors herein have identified potential issues with such approaches. As one example, since such operations to reduce turbo lag are performed only when driver tip-in is imminent, turbine speed is allowed to decrease significantly during the deceleration and hence, energy to increase turbine speed is greater. Thus, fuel injection is required to provide the extra exhaust energy to spin-up the turbine, which increases fuel usage and degrade fuel economy. Further, while the overall air-fuel ratio may be maintained at stoichiometry with the second injection, the unburned fuel in the exhaust may react with the exhaust catalyst and increase a temperature of the exhaust catalyst, thereby increasing a risk of thermal degradation of the exhaust catalyst. Still further, in order to control engine speed while increasing turbine speed, it may be required to limit the power generated during combustion by air flow adjustments and/or spark retard, for example, which may degrade engine efficiency in addition to degrading fuel economy.
Thus, in one example, some of the above issues may be at least partially addressed by a method for a boosted engine, comprising: in response to a deceleration event, deactivating fuel injectors to all cylinders of the engine while increasing airflow through a turbine of a turbocharger when a temperature of an exhaust catalyst downstream of the turbine is between a upper threshold and a lower threshold. In this way, by increasing air flow to the turbine during selected deceleration conditions, boost response during a subsequent tip-in may be improved.
As one example, in response to a tip-out that occurs after a long and large tip-in, when the tip-out conditions favor a deceleration fuel shut-off event and if an exhaust catalyst temperature is within a threshold range, fuel injectors to all cylinders of the engine may be deactivated. Simultaneously, airflow through a turbine may be increased (e.g., by opening an air intake throttle, closing wastegate, etc.), resulting in increased turbine speed. Further, airflow through the turbine may be adjusted so as to maintain the turbine speed above a threshold speed. Thus, during the tip-out, due to increased airflow to the turbine, turbine speed is not allowed to drop below the threshold. By maintaining turbine speed above the threshold, time taken to spool up the turbine to a desired speed during a subsequent tip-in is reduced. Consequently, turbo lag is reduced and boost response is improved.
Further, air flow through the turbine may be increased only when the tip-out occurs after a long and/or large tip-in. Thus, the turbine speed at the start of deceleration fuel shut-off is sufficient to enable faster increase of turbine speed above the threshold with reduced actuator adjustments. Further, by increasing air flow only when the fuel injectors are deactivated, measures to limit power generated during combustion (e.g., spark retard) may not be utilized, which improves fuel economy and engine braking efficiency. Still further, by increasing air flow during deceleration fuel shut-off conditions only when the exhaust catalyst temperature is within a threshold range, exhaust catalyst efficiency may be improved and thermal damage due to excess exhaust heat may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.